Tillerson's comments come during donor conference in Kuwait to support reconstruction and stabilisation efforts in Iraq.

The fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group is far from over, the US secretary of state has said, urging unity among Arab countries in the Gulf for the sake of regional stability.

Rex Tillerson says the rift between Qatar and its former allies in the Gulf Cooperation Council (GCC) - namely Saudi Arabia, the UAE and Bahrain, as well as Egypt - is counterproductive to efforts to root out the group.

"The end of major combat operations does not mean we have achieved the enduring defeat of ISIS," Tillerson said during a meeting on Tuesday in Kuwait City.

He said US priorities are to prevent ISIS from establishing a so-called caliphate in Iraq and Syria, recruiting new members and being able to train for future attacks.

"Without continued attention and support from coalition members, we risk the return of extremist groups like ISIS in liberated areas of Iraq and Syria and their spread to new locations," he said.

The US also pledged $200m to support stabilisation efforts in Syria, Tillerson announced.

Ibrahim Fraihat, professor of conflict resolution at the Doha Institute for Graduate Studies in Qatar, said Tillerson was "sending a message that the two tracks cannot be disconnected: that the war on terrorism can be affected by instability in the Gulf".

Iraq reconstruction

Tillerson's comments came during a three-day donor conference in Kuwait to support reconstruction and stabilisation efforts in Iraq.

Experts estimate $88bn is needed to fund the reconstruction efforts, and, of that, $22bn is urgently needed to begin the rebuilding process, said Fraihat.

Large parts of Iraq's second-largest city, Mosul, which was long held by ISIL fighters, are still in ruins more than six months after it was retaken by the Iraqi government.

"The US role is always key in any process, especially in Iraq. This whole thing started back in 2003 with the American invasion of Iraq and the regime change the US caused there," Fraihat said.

So far, the groups leading the reconstruction process in Iraq have been the European Union, World Bank, UN, Kuwait and the Iraqi government itself, he said.

"The US should be taking a leading role in this, whether it's ... in terms of financing, or in terms of providing the support for this reconstruction process."